With the effects of global warming becoming increasingly evident the need for alternative, renewable energy sources is becoming ever more acute. Much research has gone into wind, solar, and geothermal energy sources with the result that the cost of wind power per kilowatt hour has come down to the point where it is essentially on a par with energy from oil and is in fact cheaper than nuclear energy. Even the relatively high cost of solar energy is starting to see substantial price improvements.
However, the cost advantage with respect to wind and solar energy is somewhat eroded when looking at small scale energy production. This can best be explained by noting that there are two components to providing the energy to the consumer: the first is the cost to produce the energy, and the second is the synchronizing of the energy to the grid. For instance in the case of large scale wind energy (1.3 MW and more) the cost to produce is approximately $0.5/Watt and the cost to synchronize to the grid is also about $0.5/Watt. In contrast, in the case of small scale wind energy production (up to about 500 W), e.g. using smaller windmills instead of large wind generators, the cost of production is about twice as much (about $1/Watt) and for photovoltaic (PV) modules it is $2.30 to $3.50/W. In addition, the cost of synchronizing to the grid is significantly higher at about $3.2/Watt. Thus small scale wind and solar energy is currently cost prohibitive due to the cost of converting the DC from the windmill or wind turbine, or from the solar panels, to AC that is synchronized with the grid.
The reason for this high cost is the cost of grid tie inverters. Currently one of the cheaper grid tie inverters is the SunnyBoy, made by SMA, which carries a price tag of about $1700 for 700 Watt.
In the past a large number of photovoltaic (PV) cells were interfaced with a central inverter in order to achieve the desired voltage and current. This however requires high voltage dc cables. GTI address this issue by making use of an integrated AC module that integrates a single photovoltaic (PV) cell 100 and inverter 102 into one electrical device thereby avoiding mismatch losses between PV cells. Its modular structure thus simplifies enlargement of the system by providing a simple plug-and-play solution. As shown in FIG. 1, each module comprises a DC to AC inverter 102 that makes use of IGBTs or MOSFETs.
However, another cost factor is the use of large transformers with large coils. For instance, in order to boost 24V DC to 110V AC at a frequency of 50 Hz, V=L dI/dt tells us that for (110-24)V and di/dt=50 Hz the inductance of the coils has to be about 2 Henrys, thus requiring extremely large and expensive copper coils. One approach to solving this issue has been the use of high frequency transformers, which allows the use of printed circuit board magnetic components.
One such prior art module is shown in FIG. 2, which shows a PV cell 200 connected to a high frequency transformer 202 with a large capacitor 204 in parallel with the PV cell 200. The secondary winding of the transformer 202 is connected to a diode bridge 210 to convert the high frequency AC back to DC. A large solenoid 212 provides smoothing of the DC output before switching the DC to the grid frequency using switchers 214. It will be appreciated that the diodes of the bridge 210 have to handle large currents and voltages and therefore are large power devices, as are the power switching transistors 214. Also, since the transformer 202 has to handle large amplitudes albeit at high frequencies it adds substantial core losses due to hysteresis in the core.
The present invention seeks to address some of these issues.